The present invention relates generally to systems for controlling engine speed in an internal combustion engine. More specifically, the invention concerns systems and methods for accurately maintaining engine speed at low idle speeds.
Engine speed control systems, commonly known as engine speed governors, are well known in the automotive industry. In one type of engine speed governor, commonly known as a cruise control, a constant vehicle speed is maintained for a user-defined input. In this cruise control, or isochronous application, the engine speed is maintained constant regardless of the torque load applied to the vehicle engine.
A typical engine speed control system is depicted in FIG. 1. Specifically, an engine 10 includes a fuel control system 12 that controls the amount of fuel provided to the engine. The speed of the engine is directly proportional to the quantity of fuel thus provided. The engine 10 can include a speed sensor 15 that produces a signal on signal line 16 corresponding to the actual engine speed, N.sub.ACT. In a typical engine, engine speed is measured using a pulse train generated by a toothed tone wheel and magnetic pickup arrangement. The magnetic pickup signal is pre-processed by an analog circuit that converts the signals into a pulse train. This pulse train is then fed to a counter/timer, typically included within an engine control module (ECM) 20. This counter/timer calculates the elapsed time between tone wheel pulses, and the angular velocity is calculated as the known angular spacing between teeth divided by the elapsed time. For use in various engine control routines, the result of this operation can be further conditioned to produce the actual engine speed signal N.sub.ACT. Details of a suitable engine speed sensor system can be found in U.S. Pat. No. 5,165,271, which disclosure is incorporated herein by reference.
In accordance with this engine control system, an engine control module 20 receives a variety of inputs, including the engine speed signal N.sub.ACT. In addition, the ECM 20 receives a second signal on line 28 that is produced by a throttle position sensor 29. More particularly, the throttle position sensor 29 translates the position of the vehicle accelerator pedal to a requested engine speed, N.sub.REF.
The ECM 20 can include a memory 23 which stores a variety of algorithms and constants necessary for determining the operating conditions of the engine 10. The ECM 20 also includes a fuel control module 25 that receives the N.sub.ACT signal 16, the N.sub.REF signal 28 and data from the memory 23 to determine an appropriate fueling command to be provided to the fuel system 12. In particular, the fuel control module 25 incorporates the engine speed governor that operates to modulate the fuel control signal 26 as a function of the difference between the actual engine speed N.sub.ACT and the expected engine speed N.sub.REF.
One such engine speed control system is shown in U.S. Pat. No. 5,553,589, owned by the assignee of the present invention. The '589 Patent shows one type of speed control system that includes a variable droop feature. The general components of the speed control system in the '589 Patent, along with other prior art speed governors, is depicted in the control system block diagram FIG. 2. It is understood that the representations in FIG. 1 and 2 of this prior engine speed governor are relatively generic and for illustration purposes only. Specific details of the speed control system of the '589 Patent are left to the specification and figures of that patent, which information is incorporated herein by reference.
Turning now to FIG. 2, a fuel control module 25' is depicted. Specifically, the module 25' receives the reference speed signal N.sub.REF, which is based upon the user controlled throttle position. The actual engine speed N.sub.ACT is provided on signal line 16 to a summing node 30. Specifically, the actual engine speed N.sub.ACT is subtracted from the reference speed value N.sub.REF to produce a speed error signal 31, N.sub.ERR. This error signal 31 is indicative of the difference between the desired engine speed and the actual engine speed. This signal 31, N.sub.ERR is provided to a linear controller 32 that applies a transfer function C(s) to the error value. This linear controller can be of a variety of types, but most preferably is a PID controller. In the typical engine control system, the linear controller 32 generates a fuel control signal 26' that is provided to the fuel system 12 of the engine 10. In the prior systems, this fuel control signal corresponds to a degree of actuation of a flow control valve forming part of the fuel control system. In a typical installation, the fuel control signal 26' corresponds to a particular volume of fuel per stroke of the fuel control valve.
This fuel control signal 26' is provided to the engine 10, which can be approximated in the control system diagram of FIG. 2 by a transfer function G(s). The engine transfer function G(s) translates the fuel control signal 26' to an actual engine speed N.sub.ACT.
For any engine speed control system, the engine 10 can be approximated by a transfer function G(s) as represented in the control system diagram of FIG. 3. In particular, the fuel system 12 of the engine can be represented by a fuel system delay 13. The delay 13 receives the fuel control signal 26 and translates that signal to a supply of fuel to the engine after a time delay L. This delay corresponds to the activation of the mechanical and fluid components of the fuel control system 12. The combustion process can be represented by the transfer function k in block 14. Specifically, the value k corresponds to the translation of fuel to engine torque produced by combustion of the fuel within the engine. In one specific example, the transfer function k can have a value of 5.1.
While the combustion of the fuel generates torque within the engine, this torque bears a predetermined relationship to the actual engine speed. Specifically, the torque load applied to the engine is subtracted from the torque generated at block 14 at summing node 19. This combined torque is then converted to rotational speed as a function of the inertia of the rotating components of the engine, which is represented by block 17. In an ideal engine, the actual engine speed N.sub.ACT would be a function of only those components. However, the rotating components of the engine generate a certain amount of friction torque, which is known to be a function of the engine speed. Thus, a transfer function C(N) is introduced at block 18 in a feedback loop from the output of block 17 to the summing node 19. This friction torque value is thus subtracted from torque load and the torque produced by combustion of the fuel.
The friction torque represented by the transfer function C(N) is a non-linear function of engine speed. Thus, it is known that one typical speed-torque curve has a hyperbolic shape centered on a specific low engine speed. Above that engine speed, the torque gradually increases. Below that engine speed, the friction torque dramatically increases. This great increase in friction torque is primarily due to the fact that at the lower engine speed oil pressure is lower, which means that less oil is circulating between the rotating components of the engine.
The friction torque transfer function C(N) can be approximated by an equation as a function of engine speed N using a linear regression analysis. Although the torque vs. speed curve is non-linear, the system can be linearized using a differential equation based upon the incremental change in engine speed due to a incremental change in commanded fueling to the fueling system 12. Thus, the following equation can be developed to simulate the engine 10, based upon the block diagram of FIG. 3: ##EQU1##
The DC gain of the linearized system is a ratio of the coefficients applied to .DELTA.F.sub.STROKE and .DELTA.N. In otherwords, the DC gain of the typical linearized system for controlling engine fueling can be represented by the following equation. ##EQU2##
As with most control systems, the sign of the DC gain can change. As depicted in FIG. 5, the DC gain is a function of engine speed for the linearized system represented by the above equations is indicated by the curves 25'. The DC gain changes sign in the illustrated embodiment at an engine speed of approximately 460 rpm. The linearized system has a negative feedback when the engine speed is above 460 rpm. On the other hand, the system has a positive feedback when the speed is below 460 rpm.
As the curves 25' demonstrate, this known engine speed control system becomes unstable when operated below a particular engine speed, in this case about 600 rpm. These effects can be a result of the non-linear relationship between engine speed and friction torque, for instance. According to the DC gain equation (2), the partial differential of the friction torque relative to engine speed ##EQU3##
is in the denominator of the ratio. Thus, at a particular engine speed, the partial differential can equal zero, which means that the gain approaches infinity. In addition, at a lower engine speed, the sign of the partial differential will change, as reflected by the portion of the curve 25' below 460 rpm.
As the DC gain curve 25' of FIG. 5 demonstrates, the system is stable at engine speeds above about 600 rpm (although, this stability point may vary depending upon the particular engine and its friction torque feature). Thus, this prior engine speed control system performs very well at normal operating speeds--i.e. when the engine speed is in excess of 600 rpm.
A problem arises when the engine is to be operated at an idle condition. It is frequently desirable to have an idle speed that is below the speed at which the traditional linear controller is capable of sustaining. In the illustrated example of FIG. 5, the low limit speed is about 460 rpm, although operation at speeds below 600 rpm exhibits severe instability. At speeds below 600 rpm the engine speed control system is increasingly less capable of maintaining a constant speed. Thus, if it is desired to run the engine at a speed below the first threshold of 600 rpm, the operator must endure significant variations in engine speed.
More significantly, if it is desired to run the engine below the lowest threshold speed, namely 460 rpm, the traditional engine speed control system is incapable of operating in that manner. As indicated by the equations above, these limitations are not specifically imposed by the linear controller, such as controller 32 (see FIG. 2). Instead, these limitations are imposed by the dynamics of the engine 10 itself, and more specifically by the partial differential ##EQU4##
There is therefore a significant need for an engine speed control system that is capable of sustaining stable speed control at low engine speeds. Moreover, this need extends to such a system that can account for dynamic changes between different engines.
One important link in an engine speed control system is the accuracy of the engine speed signal. In a typical system, as depicted in FIG. 7, a tone wheel 50 is driven from the engine camshaft. The tone wheel includes a plurality of teeth 52 evenly distributed around the circumference of the tone wheel at known angular intervals. A typical tone wheel includes 24 teeth 52. An additional tooth 54 can be provided to identify a top-dead-center position of a particular reference cylinder.
A sensor 60 is arranged to generate a signal 62 as each tooth passes. The signal 62 is fed to an amplifier and shaping circuitry 65 that can transform the analog signal to a squared pulse train signal. In one embodiment, this pulse train signal is fed to the ECM 20, which includes circuitry or software to calculate the engine speed from the pulse train information. One engine speed sensor configuration is disclosed in U.S. Pat. No. 5,165,271, which description is incorporated herein by reference.
For the purposes of the present disclosure, the circuitry 65 has been presumed to encompass circuitry and/or software modules normally resident within the ECM 20 that is needed to generate the speed signal 16 discussed above. These modules can include a timer and signal sample module that count the elapsed time between tooth passages, and then divides the angular distance between teeth by that elapsed time to generate the engine speed signal. In control circuit terms, the traditional speed sensor is a zero order sample and hold component with a half time delay.
Due to sensor eccentricity and tooth error, a sampled speed signal can contain disturbances superimposed over the pulse train. Typically, these errors are at the shaft speed and can lead to replicated frequency signals at multiples of the sensed and discretized signal. These replicated frequency components can be referred to as frequency aliasing. Under many conditions, this frequency aliasing does not pose a significant problem to the integrity of the speed sensor data and speed signal. However, in some circumstances, such as within a multi-sampling scheme in a speed control loop, the replicated frequencies of the disturbances are introduced into the low frequency domain where the speed control algorithms operate. Thus, the low frequency aliasing can cause surging and unwanted speed variation.
This problem can be particularly troublesome in a vehicle speed control system. The components of the vehicle speed control system can be similar to those depicted in FIG. 7, except that the tone wheel 50 is associated with the vehicle tailshaft. In one example, sampling disturbances can occur at about 34 Hz. Due to the frequency aliasing phenomena, these disturbances can be shifted to less than one Hz, which is sufficiently low to be active during the speed control processing.
One approach to addressing this signal noise problem is to utilize higher quality components--i.e., tone wheel and sensor. Of course, higher quality usually means greater expense. Another approach is to reduce the speed sampling rate to once per revolution. This approach introduces a large sampling delay and limits the sample rate, which translates to poor speed control performance, especially at low engine or vehicle speeds.
A need exists for a speed control system that eliminates or suppresses frequency aliasing that is introduced in a multi-sampling scheme, without sacrificing the sampling rate benefits. This need can best be served by a system that does not rely upon additional or more expensive hardware or circuitry.